1. Field of the Invention
This invention relates to a method of preparing a hard disc.
2. Prior Art
In the prior art, hard discs are generally manufactured by forming a nickel--phosphorus (Ni--P) coating on a surface of an aluminum or aluminum alloy substrate from an electroless nickel--phosphorus plating bath containing hypophosphorous acid or a salt thereof as a reducing agent. Prior to the formation of a Ni--P film, the substrate must be subject to zinc immersion coating or zinc replacement treatment to form a zincate film on the substrate surface.
Conventional immersion zincate solutions contain zinc compounds and alkali hydroxides. When immersion zinc coating is carried out using such zincate solutions, there is deposited a zincate film consisting of thick and thin portions and presenting uneven surface topography which is sometimes referred to as zinc replacement variation. Then an electroless Ni--P coating formed subsequent to the zincating treatment presents a surface of the same topography as the zincate film. This plating variation is created by a collection of nodules. Even after polishing, the area of plating variation can be left as an area having minute projections of the order of 20 to 50 nm. As the recording density of hard discs increases, such minute projections become a bar.
It is thus desired to preclude plating variation which would occur when a Ni--P film is formed on an aluminum or aluminum alloy substrate from an electroless plating bath, that is, to preclude zinc replacement variation causing such plating variation.